Minerals within a crater in Nili Fossae, Mars

We are looking at an outcrop within the center of an unnamed 40-kilometer-diameter crater within Nili Fossae on Mars. The base map is a CTX image; superimposed on it is a mineral map derived from CRISM infrared spectra. The inset shows spectra for two minerals observed in the CRISM data: smectite, a clay mineral containing iron and magnesium (magenta line and magenta areas on map) and kaolinite, a clay mineral that does not contain iron or magnesium (green line and green areas on mineral map). Examination of the relationship between the CRISM mineral map and the base CTX image shows that most of the thick sedimentary units filling the crater contain smectite (magenta), but they are overlain by a distinct, bright-toned kaolinite-bearing material (green). Both have been exposed from beneath a capping unit (purple) by erosion of the deposits by the action of liquid water.